The car bombings of the World Trade Center in New York City in February 1993 and the Alfred P. Murrah Federal Building in Oklahoma City in April 1995 are perhaps the two most devastating terrorist acts in the United States. However, there are many less publicized criminal bombings. According to the Wall Street Journal (Aug. 2, 1996), there were 1,573 bombings and bomb attempts in the U.S. in 1990. Over the years this number has steadily grown to 2,438 in 1994, the last full year for which statistics are available. This is an increase of over 10% per year. It is clear that effective techniques have to be devised to improve the security of buildings against the effects of criminal bomb blasts.
The current techniques to enhance building security are: 1) detection and prevention, 2) keep-out distance and 3) structural modifications to increase ductility, redundancy, and load path.
The economic and social costs of detection and prevention on a routine basis include intrusions on individual privacy and curtailment of people's movements, which is not possible in an open society.
For a given charge weight (equivalent amount of TNT), the larger the keep-out distance, the less would be the blast load on the structure. However, many public and private buildings are located in metropolitan areas where the cost of real estate is high, and most often the keep-out distance is limited to the public sidewalk.
Structural modifications to increase ductility, redundancy and load path invariably involve structural stiffening. Stiffening the structure reduces its fundamental natural period. Reducing in the fundamental period of a structure would increase the level of the blast load that the structure can experience.
Another technology that could possibly be used to mitigate some of the blast effects is to increase the fundamental natural period of the structure via seismic base isolation technology. However, due to the variety of structural components and their response modes, and the uncertainty of the frequency and magnitude of the blast loads, it is not clear how effective seismic isolation would be. This is an area that merits further investigation. Of course there are other means of changing the natural period of structures and structural elements.
Here an alternative technology is proposed which does not have any of the limitations of the current techniques enumerated above. The proposed technology involves covering each face of an exposed wall with a very flexible and inflatable double layer membrane. When inflated, the flexible membranes are vented to each other through holes in the wall. There are also pliable cover walls which protect the membranes against explosion-generated projectiles. A structure utilizing the proposed technology 1) attracts a much smaller fraction of the blast load due to its large flexibility; 2) by venting the blast pressure from the front of a structural element, such as a panel, to its back, reduces the load which are to be resisted by the structural element and the structure as a whole; 3) can protect people and equipment from flying projectiles generated by spalling of the surfaces of the walls.
The proposed technology does not have any of the limitations of the current techniques. It can be used to mitigate blast effects on external walls as well as internal walls such as the ones in the underground parking lots. It can also be used for new as well as existing structures.